The present invention relates to a heat treatment apparatus for thin spheroidal graphite cast iron products.
Recently, attempts have been made to produce thin cast iron products for the purpose of improving mechanical strength. However, since melts of spheroidal graphite cast iron are quickly cooled in casting molds in the case of thin cast iron products, cementite phases are likely to appear. The thin cast iron products of spheroidal graphite cast iron in which cementite phases are precipitated are poor in cuttability, elongation and impact strength. Accordingly, they are easily broken. To overcome these problems, thin cast iron cooled to low temperatures such as room temperature, are heated again to temperatures equal to or higher than their A.sub.3 transformation points, for instance, 850.degree.-950.degree. C. By this heat treatment, cementite is decomposed and pearlite in the matrix is ferritized.
However, when this heat treatment is conducted on thin spheroidal graphite cast iron products, primarily precipitated spheroidal graphite particles (hereinafter referred to as "primary spheroidal graphite particles") are diffused in the matrices, leaving fine gaps around their graphite particles. As a result, the thin spheroidal graphite cast iron products inevitably have reduced mechanical properties, particularly fatigue strength.
In addition, since the spheroidal graphite cast iron products are heated to a high temperature after cooled to room temperature, a large amount of thermal energy is consumed, meaning that this process is economically disadvantageous.
As a method for solving these problems, the applicants previously filed a patent application in Japan for a method of producing a thin high-strength article of spheroidal graphite cast iron having spheroidal graphite particles dispersed in a ferrite matrix containing 10% or less of pearlite with substantially no fine gaps between the spheroidal 10 graphite particles and the ferrite matrix, by pouring a melt having a spheroidal graphite cast iron composition into a casting mold; removing the casting mold by shake-out after the completion of solidification of the melt, while substantially the entire portion of the resulting cast iron product is still at a temperature of its A.sub.3 transformation point or higher; immediately introducing the cast iron product into a uniform temperature zone of a continuous furnace kept at a temperature of the A.sub.3 transformation point or higher, where the cast iron product is held for 30 minutes or less to decompose cementite contained in the matrix; and transferring the cast iron product into a cooling zone of the continuous furnace to cool the cast iron product at such a cooling speed as to achieve the ferritization of the matrix. (Japanese Patent Application No. 1-234485 claiming domestic priority of Sept. 9, 1988, corresponding to U.S. Ser. No. 403,876).
Thus, an apparatus for efficiently conducting the above method has been desired.